The molecular basis of R plasmid maintenance in E. coli will be studied by (1) determining the role of host and plasmid genes in the replication of plasmid DNA and (2) the regulation of plasmid genes at the transcriptional level. A physical map of the R plasmid (R538-1) will be constructed by mapping the fragments produced by the eco Rl restriction endonuclease. After the map has been constructed, individual fragments will be hybridized with pulse-labeled RNA to produce a transcription map of the R plasmid. Regulation of transcription will be determined by measuring RNA hybridization in mutants repressed for transfer, and in mutants which overproduce R plasmid DNA. The effect of antibiotic concentration on the level of transcription and the level of antibiotic inactivating enzymes will also be determined. Experiments investigating the mechanism of Pl plasmid maintenance in lysogenic cells, and the role of phage and host genes in this maintenance will also be carried out.